How migratory thrushes conquered northern North America: a comparative phylogeography approach

Abstract

Five species of migratory thrushes (Turdidae) occupy a transcontinental distribution across northern North America. They have largely overlapping breeding ranges, relatively similar ecological niches, and mutualistic relationships with northern woodland communities as insectivores and seed-dispersing frugivores. As an assemblage of ecologically similar species, and given other vertebrate studies, we predicted a shared pattern of genetic divergence among these species between their eastern and western populations, and also that the timing of the coalescent events might be similar and coincident with historical glacial events. To determine how these five lineages effectively established transcontinental distributions, we used mitochondrial cytochrome b sequences to assess genetic structure and lineage coalescence from populations on each side of the continent. Two general patterns occur. Hermit and Swainson's thrushes (Catharus guttatus and C. ustulatus) have relatively deep divergences between eastern and western phylogroups, probably reflecting shared historic vicariance. The Veery (C. fuscescens), Gray-cheeked Thrush (C. minimus), and American Robin (Turdus migratorius) have relatively shallow divergences between eastern and western populations. However, coalescent and approximate Bayesian computational analyses indicated that among all species as many as five transcontinental divergence events occurred. Divergence within both Hermit and Swainson's thrushes resembled the divergence between Gray-cheeked Thrushes and Veeries and probably occurred during a similar time period. Despite these species' ecological similarities, the assemblage exhibits heterogeneity at the species level in how they came to occupy transcontinental northern North America but two general continental patterns at an among-species organizational level, likely related to lineage age.

Keywords: Community assembly; Community phylogeography; Ecological evolutionary genetics; Ecology; Phylogeography; Population genetics.

Figure 1. Thrush distribution maps.

Maps of species breeding ranges with sample locations shown: Hermit Thrush (A), Swainson’s Thrush (B), Gray-cheeked Thrush (C), Veery (D), and American Robin (E). Maps are based on the Birds of North America Online http://bna.birds.cornell.edu/bna and AOU (1998). Black circles are proportional in size to the number of individuals sampled from each location. Locations are eastern: Nova Scotia (NvSc) and Newfoundland (Newf); and western: Interior Alaska (AK), Southeast Alaska (SE AK), Hyder, Alaska (Hyder), Queen Charlotte Islands (QCI), and Washington state (WA).

Figure 2. Haplotype networks and trees.

Statistical parsimony networks showing haplotype relationships and the number of individuals with each haplotype. Shading indicates general sampling areas in North America; black, eastern; white, western; and gray, Hyder, Alaska. The size of each circle is proportional to the number of individuals with each haplotype. The length of connecting lines is proportional to the number of base pair differences between haplotypes. The phylograms on the right are sized proportionally to each other. Phylogeographically important nodes with Bayesian posterior probabilities of 1.0 are shown with an asterisk. Inset phylogram (F) shows the Gray-cheeked Thrush (top clade) and Veery (bottom clade) combined.

Figure 3. Divergence time estimates in graphic form.

Divergence time in years before present converted from smoothed IM values of divergence times with 95% credible intervals for t (white bars) and TMRCA (gray bars), assuming a one-year generation time and 2% sequence divergence per million years for cyt b in passerines. Along the bottom is a geological time scale showing different segments of the Pleistocene and the Wisconsin glacial period (W). The last glacial maximum (∼18,000 ybp) is shown with a thick line within the late Wisconsin. An asterisk indicates that the upper 95% credible interval value was used from TMRCA (see methods).

Figure 4. msBayes results.

msBayes posterior probability graphs of Ψ (Psi) for the five-taxon dataset (all five thrush species east–west populations) and the three ‘taxon’ dataset (Hermit Thrush and Swainson’s Thrush phylogroups, and Gray-cheeked Thrushes and Veeries combined).